The reference "Bedienungshandbuch zum Schutzgerat" (Protection Device Operating Manual), 7SA513 V3.1, Siemens AG, p. 68, describes a method in which phase currents and phase voltages are sampled to forming phase current and phase voltage sampling values. An impedance vector is formed from these sampling values and tested for its rate of change in the complex R-X impedance plane. A signal identifying a three-pole short-circuit is generated if the rate of change of the impedance vector in the R-X impedance plane is excessively high.